During the mid to late 1800s, scientists gained a greater understanding of the sources and effects
of drinking water contaminants, especially those that were not visible to the naked eye. In 1855,
epidemiologist Dr. John Snow proved that cholera was a waterborne disease by linking an outbreak of
illness in London to a public well that was contaminated by sewage. In the late 1880s, Louis Pasteur
demonstrated the “germ theory” of disease, which explained how microscopic organisms (microbes)
could transmit disease through media like water.